Carbon fiber powder, a method of making the same, and thermally conductive composition

ABSTRACT

A carbon fiber powder is produced by graphitizing a polymeric fiber having an aromatic ring on its main chain by heating and then pulverizing or cutting it. The polymeric fiber is selected from the group consisting of polybenzazole fiber, aromatic polyamide fiber, aromatic polyimide fiber, polyphenylene sulfide fiber, and wholly aromatic polyester fiber. The carbon fiber is preferably graphitized by heating the polymeric fiber at least 2500 degree C. under vacuum or in an inert gas. Thus, the carbon fiber powder that has excellent thermal conductivity and filling capability, a method of making the carbon fiber powder, and a thermally conductive composition including the carbon fiber powder are provided.

BACKGROUND OF THE INVENTION

[0001] This invention relates to carbon fiber powder that has excellentthermal conductivity and filling capability, a method of making thesame, and a thermally conductive composition containing the powder.

[0002] With recent advancements, miniaturization, and lightening ofelectronic hardware, semiconductor packages have become more compact andmore highly integrated and operated at higher speed. Therefore, the heatgenerated by the electronic hardware is a very important issue.Generally, to dissipate the heat from heat-generating components tooutside, a molded article and a liquid composition, such as a sheetmaterial made of a thermally conductive polymer composition, polymergrease and adhesive, are placed between a radiator and one of thefollowings: a printed circuit board; a semiconductor package; and a heatradiator. The heat radiator is, for example, a radiation plate or a heatsink.

[0003] Such thermally conductive compositions include a matrix, such asresin and rubber, and a filler that has high thermal conductivity in thematrix. Possible fillers include metal oxide, metal nitride, metalcarbide, and metal hydroxide. Examples of such possible fillers includealuminum oxide, boron nitride, silicon nitride, magnesium oxide, zincoxide, silicon carbide, quartz, and aluminum hydroxide. However, suchcompositions do not necessarily have sufficient thermal conductivity.

[0004] In order to improve the thermal conductivity, variouscompositions have been proposed that include highly thermally conductivegraphite powders or carbon fibers as filler in the matrix.

[0005] For example, Japanese Laid-Open Patent Publication No.62-131033discloses a molded body made of thermally conductive resin in which theresin is filled with graphite powders. Japanese Laid-Open PatentPublication No.4-246456 discloses a composition of polyester resincontaining carbon black or graphite. Japanese Laid-Open PatentPublication No.5-17593 discloses a thermally conductive molded body ofgreat mechanical strength in which the carbon fibers are arranged in acertain direction and are impregnated with graphite powder andthermosetting resin. Japanese Laid-Open Patent Publication No.5-222620discloses a thermally conductive material using pitch-based carbonfibers that have a specific cross section. Japanese Laid-Open PatentPublication No.5-247268 discloses a rubber composition in which is mixedsynthetic graphite having a particle size of 1 to 20 μm. JapaneseLaid-Open Patent Publication No.9-283955 discloses a thermallyconductive sheet in which the graphitized carbon fibers of specificaspect ratio are dispersed in polymer, such as silicone rubber. JapaneseLaid-Open Patent Publication No.10-298433 discloses a composition and aradiation sheet in which silicone rubber has, mixed within it, sphericalgraphite powders having an interplanar spacing of crystals from 0. 330to 0. 340 nm. Japanese Laid-Open Patent Publication No.2-242919 andJapanese Laid-Open Patent Publication No.7-48181 disclose certainpitch-based carbon fibers as highly thermally conductive carbon fibers.

[0006] Thus, pitch-based carbon fiber has been known for highlythermally conductive carbon fiber used as thermally conductive filler.More particularly, isotropic pitch-based carbon fiber and mesophasepitch-based carbon fiber have been known for that purpose. Suchpitch-based carbon fiber is obtained by spinning raw pitch,infusibilizing the resulting pitch fiber, and graphitizing it byheating. On the other hand, highly thermally conductive fiber cannot beobtained from PAN-type carbon fibers and rayon-type carbon fibersproduced by graphitizing polyacrylonitrile fiber and rayon fiber byheating, since the graphitization upon heating is hard to achieve. Tothis end, a pitch-based carbon fiber is advantageous.

[0007] Meanwhile, Japanese Laid-Open Patent Publication No.9-324127discloses highly thermally conductive powdery graphite, die bondadhesive for a semiconductor element, and a semiconductor device inwhich the powdery graphite is blended in thermosetting adhesive resin.The powdery graphite is obtained by graphitizing a polymeric film byheating and pulverizing or cutting the resultant graphitized film.

[0008] However, for recent high performance electronic parts, due toincrease in amount of heat generation, the need for greater thermalconductivity has increased. Therefore, the thermal conductivity is stillinsufficient for the above-mentioned compositions that include graphitepowder or carbon fibers as thermally conductive filler.

[0009] Further, as for pitch-based carbon fibers, pitch fiber used as araw material has poor heat resistance. Therefore, to prevent pitch fiberfrom melting at high temperature, infusibilizing for several hours below400 degree C. with oxygen-containing gas such as air, placing inoxidative water solution such as nitric acid and chromic acid, orpolymerizing with light or gamma ray, are required before thegraphitization process by heating. This leads to low productivity of thefiber.

[0010] The powdery graphite proposed in Japanese Laid-Open PatentPublication No.9-324127 has an advantage that the infusibilizationprocess is eliminated. However, since a planar polymeric film is used asa raw material of the powdery graphite, cumbersome pulverization orcutting of the film is needed after heat treatment. In addition, theresultant pulverized products or cut products have non-uniform shape orsize. In other words, fine powders and coarse powders are present in theproducts. Thus, it is difficult to contain such powdery graphite fillerin a matrix at high concentration.

[0011] The objective of the present invention is to provide carbon fiberpowder that has excellent thermal conductivity, as well as is filled ina matrix at high concentration; a simple method of making the carbonfiber powder; and a thermally conductive composition including thecarbon fiber powder.

BRIEF SUMMARY OF THE INVENTION

[0012] According to the present invention, a carbon fiber is produced bygraphitizing a polymeric fiber having an aromatic ring on its main chainby heating. The carbon fiber may be in the powder form.

[0013] Other aspects and advantages of the invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Embodiments of the present invention are described in detailbelow.

[0015] 1. Carbon Fiber Powder

[0016] The carbon fiber powder is a pulverized or cut product of thecarbon fiber that is obtained by graphitizing by heating a polymericfiber having an aromatic ring on its main chain.

[0017] 1.A. Polymeric Fiber as a Raw Material

[0018] Firstly, a polymeric fiber of the present invention used as a rawmaterial is described. The polymeric fiber of the present invention hasan aromatic ring on its main chain. As used herein, an aromatic ringgenerally refers to a ring that belongs to an aromatic group and means agroup of organic compounds including aromatic hydrocarbons, such asbenzene ring, naphthalene ring, and anthracene ring, and derivativesthereof.

[0019] The polymeric fiber of the present invention is not particularlylimited to, but includes, polybenzazole fiber, aromatic polyamide fiber,aromatic polyimide fiber, polyphenylene sulfide fiber, wholly aromaticpolyester fiber, polyphenylene benzoimidazole fiber, polyparaphenylenefiber, and polyparaphenylene vinylene fiber.

[0020] One of the reasons for using the polymeric fiber having anaromatic ring on its main chain as a raw material is that such apolymeric fiber is easily graphitized upon heating and thus produceshighly thermally conductive carbon fiber or carbon fiber powder, inwhich graphite structure is highly developed. On the other hand, when aconventional polymeric fiber having no aromatic ring on its main chainis used as a raw material, for example, polyacrylonitrile fiber, rayonfiber, or phenol fiber, the graphitization upon heating is difficult.Thus, highly thermally conductive carbon fibers or carbon fiber powdercan not be obtained.

[0021] Another reason for using the polymeric fiber of the presentinvention as a raw material is that such polymeric fiber has so highheat resistance that it is hard to melt but often keeps its shape uponheating, and thus the above-mentioned infusibilization process is notnecessarily needed. Thus, productivity at a manufacturing step isimproved.

[0022] A further reason for using the polymeric fiber of the presentinvention as a raw material is that pulverization or cutting is easierwhen using such fiber rather than using the planar graphite film as araw material. Besides, fine and uniform carbon fiber powders having alimited bulk can be easily obtained.

[0023] Among them, the polymeric fiber of the present invention is morepreferably at least one fiber selected from the group consisting ofpolybenzazole fiber, aromatic polyamide fiber, aromatic polyimide fiber,polyphenylene sulfide fiber, and wholly aromatic polyester fiber. Mostpreferably, the polymeric fiber is at least one fiber selected from thegroup consisting of polybenzazole fiber, aromatic polyimide fiber, andaromatic polyamide fiber. The polymeric fiber having an aromatic ring onits main chain tends to be graphitized easier upon heating as it hasmore aromatic rings. Thus, carbon fiber or carbon fiber powder that hasextremely excellent thermal conductivity is obtained by using suchpreferred polymeric fibers. The diameter, profile of cross section, andlength of the polymeric fiber of the present invention is not limited.

[0024] As used herein, polybenzazole fiber refers to a polymeric fiberthat is formed of polybenzazole polymer. The polybenzazole fiber isgenerally excellent in strength, modulus of elasticity, heat resistance,flame resistance, and electric insulation. The polybenzazole polymer(PBZ) refers to polybenzooxazole homopolymer (PBO), polybenzothiazolehomopolymer (PBT); or random copolymer, sequential copolymer, blockcopolymer, or graft copolymer of PBO and PBT. PBZ may be synthesized bya known method. An example of commercially available PBZ is ZYLON™ fromToyobo Co., Ltd.

[0025] The aromatic polyamide fiber includes poly(paraphenyleneisophthalic amide) and poly(metaphenylene isophthalic amide). Examplesof commercially available aromatic polyamide fiber are, forpoly(paraphenylene isophthalic amide), KEVLAR™ from Du Pont-Toray Co.,Ltd., Technora™ from Teijin Limited, and Twaron™ from Akzo; and forpoly(metaphenylene isophthalic amide), NOMEX™ from Du Pont-Toray Co.,Ltd., CONEX™ from Teijin Limited, and Apyeil™ from Unitika Ltd. Thearomatic ring of these products may include a substituent such ashalogen group, alkyl group, cyano group, acetyl group, and nitro group.

[0026] In addition, aromatic polyimide fiber (from Inspec Fibres GmbH),polyphenylene sulfide fiber (PROCON™ from Toyobo Co., Ltd.), whollyaromatic polyester fiber (VECTRAN™ from Unitika Ltd. And ECONOL™ fromSumitomo Chemical Company Limited), polybenzoimidazole fiber(fromHoechst Celanse) are easily available.

[0027] 1.B. A Method of making Carbon Fiber Powders

[0028] Next, a method of making carbon fiber powders is described. Thecarbon fiber powders are produced by graphitizing the above-mentionedpolymeric fiber of the present invention by heating, and pulverizing orcutting the resultant carbon fiber.

[0029] The heating temperature should be at least 2500 degree C. Whenthe temperature is lower than 2500 degree C., graphitization of thefiber becomes insufficient and carbon fiber that has high thermalconductivity can not be obtained. Preferably, the heating is conductedunder vacuum or in an inert gas, such as argon gas or nitrogen gas. Whenthe fiber is not heated under vacuum or in an inert gas, the polymericfiber may be undesirably degenerated by oxidation. In practice, thefiber is preferably heated for a given time at a high temperature from2800 to 3200 degree C. in argon gas. This actively promotesgraphitization of the fiber to produce highly thermally conductivecarbon fiber in which graphite structure highly develops. The process isnot particularly limited to specific rates of heating temperature or toa specific treating period.

[0030] In heating the polymeric fiber, polymeric fibers are preferablybundled. Such bundle of the fibers allows a large amount of the fibersto be placed in a certain volume of the heating reactor. Thus, theheating treatment is conducted efficiently and the productivity isimproved. Further, by pressurizing the bundled polymeric fibers, an evenlarger amount of the fibers can be placed in a certain volume of theheating reactor, thereby improving productivity.

[0031] To pulverize or cut fibers, pulverizing machines are available,such as a Victory mill, a jet mill, and a high-speed rotation mill orcutters for chopping fibers. To make the pulverization or cuttingeffective, it is advantageous if a rotor of each machine that has bladesthat rotate at high speed to cut the fibers in a direction perpendicularto the fibers. The average length of the pulverized or cut fibers ischanged by adjusting the rotation number of the rotor or an angle of theblades. Grinding machines such as a ball mill could be used forpulverizing the fibers. However, such machines are undesirable in thatthey apply perpendicular pressure to fibers, which generates cracks inthe axial direction of the fibers. This pulverization or cutting processmay be conducted either before or during the heating of the fiber.

[0032] 1.C. Properties of the Carbon Fiber Powders

[0033] The properties of resultant carbon fiber powders are described.

[0034] The graphitized carbon fiber powders specifically take the formof fiber (including a pulverized product or a cut product that keeps thefibrous form), a scale, a whisker, a micro coil, or a nanotube. However,other forms are also applicable.

[0035] The diameter of the graphitized carbon fiber powders is notparticularly limited but is preferably 5-20 μm. The fiber powders thathave a diameter of 5-20 μm are easily produced industrially. The fiberdiameters smaller than 5 μm or larger than 20 μm decrease theproductivity of the fiber powders.

[0036] The average particle size or length of the graphitized carbonfiber powders is not particularly limited but is preferably 5-500 μm.When the average particle size of each fiber powder is smaller than 5μm, the contact of the fiber powders in the matrix is reduced and a heattransfer becomes insufficient. This reduces the thermal conductivity ofthe resultant thermally conductive composition. When the averageparticle size is larger than 500 μm, the fiber powders are too bulky tobe mixed in the matrix at a high concentration. The average particlesize can be calculated from the particle size distribution by laserdiffractometry model.

[0037] From X-ray diffractometry, it is preferred that the graphitizedcarbon fiber powders have an interplanar spacing (d002) of graphiteplanes of less than 0.3370 nm. When the interplanar spacing (d002) isless than 0.3370 nm, carbon fiber powders and a thermally conductivecomposition that have higher thermal conductivity can be achieved. Whenthe interplanar spacing (d002) is 0.3370 nm or greater, the thermalconductivity is inadequate. Accordingly, a composition that has highthermal conductivity can not be obtained by using such carbon fiberpowders as thermally conductive filler. The lower limit of theinterplanar spacing (d002) is a theoretical value of 0.3354 nm.

[0038] In the X-ray diffractometry, a diffractometry pattern of thecarbon fibers is measured by using CuK alpha as a X-ray source andhighly purified silicon as a standard material. The interplanar spacing(d002) is calculated from the peak position and half-value width of the(002) diffractometry pattern. This calculation is based on a methodpursuant to Japan Society for the Promotion of Science.

[0039] The thermal conductivity is not particularly limited butpreferably at least 400W/(m·K), more preferably at least 800W/(m·K), andmost preferably at least 1000W/(m·K).

[0040] 2. Thermally Conductive Composition

[0041] A thermally conductive composition includes the above-mentionedcarbon fiber powders in a matrix as thermally conductive filler.

[0042] 2.A. Matrix

[0043] The matrix is preferably selected according to requiredcharacteristics or applications such as a shape, hardness, mechanicalproperties, thermal properties, electrical properties, durability, andreliability of the resultant sheet. Matrix is preferably a polymericmaterial if molding capability is taken into consideration.

[0044] The polymeric material is preferably selected from thermoplasticresin, thermoplastic elastomer, thermosetting resin, and vulcanizedrubber according to required characteristics. For example, preferredthermally conductive adhesives include adhesive polymeric materials suchas epoxy resin, polyimide, and acrylic resin. Preferred moldingmaterials include thermoplastic resin, thermoplastic elastomer,thermosetting resin, and vulcanized rubber.

[0045] The thermoplastic resin includes polyethylene, polypropylene,ethylene-α-olefin copolymer such as ethylene-propylene copolymer,polymethylpentene, polyvinyl chloride, polyvinylidene chloride,polyvinyl acetate, ethylene vinyl acetate copolymer, polyvinyl alcohol,polyacetal, fluororesins such as polyvinylidene fluoride andpolytetrafluoroethylene, polyethylene terephthalate, polybutyleneterephthalate, polyethylene naphthalate, polystyrene, polyacrylonitrile,styrene acrylonitrile copolymer, ABS resin, polyphenylene ether (PPE)resin and modified PPE resin, aliphatic and aromatic polyamides,polyimide, polyamide imide, polymethacrylic acid and polymethacrylatessuch as polymethyl methacrylate, polyacrylic acids, polycarbonate,polyphenylene sulfide, polysulfone, polyether sulfone, polyethernitrile, polyether ketone, polyketone, liquid crystal polymer, siliconeresin, and ionomer.

[0046] The thermoplastic elastomer includes repeatedly moldable andrecyclable thermoplastic elastomers such as styrene-butadiene orstyrene-isoprene block copolymers and hydrogenated polymer thereof,styrenic thermoplastic elastomer, olefinic thermoplastic elastomer,vinyl chloride thermoplastic elastomer, polyester thermoplasticelastomer, polyurethane thermoplastic elastomer, and polyamidethermoplastic elastomer.

[0047] The thermosetting resin includes epoxy resin, polyimide,bis-maleimide resin, benzocyclobutene, phenol resin, unsaturatedpolyester, diallyl phthalate, silicone resin, polyurethane, polyimidesilicone, thermosetting polyphenylene ether resin and modified PPEresin.

[0048] The vulcanized rubber and analogues thereof include naturalrubber, butadiene rubber, isoprene rubber, styrene-butadiene copolymerrubber, nitrile rubber, hydrogenated nitrile rubber, chloroprene rubber,ethylene-propylene rubber, chlorinated polyethylene, chlorosulfonatedpolyethylene, butyl rubber and halide butyl rubber, fluorine rubber,urethane rubber, and silicone rubber.

[0049] In terms of temperature characteristics such as thermalresistance and electric reliability, the matrix polymer is preferably atleast one material selected from the group consisting of siliconerubber, epoxy resin, polyurethane, unsaturated polyester, polyimide,bis-maleimide, benzocyclobutene, fluororesin, polyphenylene ether resinand thermoplastic elastomer. More preferably, the matrix polymer is atleast one material selected from the group consisting of siliconerubber, epoxy resin, unsaturated polyester resin, polyimide,polyurethane and thermoplastic elastomer.

[0050] In an application for a wiring board where permittivity anddielectric loss tangent are small and frequency characteristic isrequired, fluororesin, thermosetting polyphenylene ether resin, modifiedPPE resin, and polyolefin resin are desired. To obtain a flexiblethermally conductive polymer sheet, a polymer matrix such aslow-hardness vulcanized rubber and low-hardness thermoplastic elastomermay be used.

[0051] One or more of the above polymers can be used as appropriate.Further, a polymer alloy formed of a plurality of these polymericmaterials may be used. The methods of cross-linking thermosetting resinor vulcanized rubber are not limited to thermosetting but include knownmethods such as light setting and moisture setting.

[0052] 2.B. Properties of Thermally Conductive Composition

[0053] The intended thermally conductive composition is obtained bymixing the above-mentioned carbon fiber powders with the matrix anddegassing it if necessary. In mixing, known mixing machines such as ablender, a mixer, a roller, an extruder may be used.

[0054] The content of the carbon fiber powders is preferably 1 to 800parts by weight, more preferably 5 to 500 parts by weight, and mostpreferably 20 to 300 parts by weight relative to 100 parts by weight ofthe matrix. When the content is less than 1 part by weight, the thermalconductivity of the resultant composition is lowered and radiatingproperty is decreased. When the content is more than 800 parts byweight, the viscosity of the composition is increased, which makes itdifficult to disperse the carbon fiber powders in the matrix uniformly.Also, gas bubbles are inevitably included in the matrix.

[0055] For the purpose of improvement of the surface of the carbon fiberpowders, the surface of the powders may be previously oxidized byelectrolytic oxidation or treated with a known coupling agent or a knownsizing agent.

[0056] This improves wettability or filling capability of the carbonfiber powders to the matrix or the peel strength from the matrix at theinterface.

[0057] The surface of the carbon fiber powders may also be coated withmetal or ceramics by various methods such as electroless plating;electroplating; physical vapor evaporation such as vacuum evaporation,sputtering and ion plating; chemical vapor deposition, spraying;coating; immersion; and mechanochemical method in which fine particlesare mechanically fixed on the surface of the fibers.

[0058] Other than the carbon fiber powders of the present invention, thethermally conductive polymer composition may also include otherthermally conductive materials, an incombustible agent, a softeningagent, a colorant, and a stabilizer as required. The other thermallyconductive materials include the following:

[0059] metal and ceramic such as silver, copper, gold, aluminum oxide,magnesium oxide, boron nitride, aluminum nitride, silicon nitride,silicon carbide, and aluminum hydroxide

[0060] metal-coated resin

[0061] conventional graphitized carbon fiber

[0062] non-graphitized carbon fiber

[0063] natural graphite

[0064] synthetic graphite

[0065] meso-carbon micro-bead

[0066] The carbon fibers, graphites, and beads may be in the form of,for example, spherical powder, powder, fiber, needle, a scale, awhisker, a microcoil, single-wall, or multi-wall nanotube.

[0067] In an application where electric non-conductance is particularlyrequired for a thermally conductive polymer sheet, an end product, thecomposition preferably includes at least one electrical insulativethermally conductive filler selected from the group consisting ofaluminum oxide, magnesium oxide, boron nitride, aluminum nitride,silicon nitride, silicon carbide and aluminum hydroxide.

[0068] The thermally conductive composition may be processed by, forexample, compression molding, extrusion molding, injection molding,casting molding, blow molding, blade molding, and calendering molding.When the composition is liquid, it may be processed by painting,printing, dispensing, and potting other than the above methods. Forexample, a thermally conductive molded article that has excellentthermally conductive property may be produced by processing thecomposition into a predetermined shape by the above molding methods. Athermally conductive sheet which has excellent thermal conductivity maybe obtained by using vulcanized rubber or thermoplastic elastomer whichhas low hardness as a matrix and molding the composition into a sheet bythe above molding methods.

[0069] Thus, the thermally conductive composition may be used as amaterial for liquid articles such as thermally conductive grease andthermally conductive adhesive, for both of which a high heat radiationproperty is required. Alternatively, the composition may be used as amaterial for molded articles such as printed circuit boards,semiconductor packages, heat radiation members including radiationplates and heat sink, a housing, and a thermally conductive sheet. Theconductive composition may be used to form thermally conductive greaseand thermally conductive adhesive placed between the heating element andthe heat transfer member or to form heat radiation members, such as aradiator, a cooler, a heat sink, a heat spreader, a die pad, a printedcircuit board, a cooling fan, a heat pipe, and a housing. Thus, aheat-dissipating measure is possible.

[0070] The advantages of the above embodiments are described below.

[0071] For the polymeric fiber of the present invention, the polymericfiber having an aromatic ring on its main chain is used, preferably, atleast one polymeric fiber selected from the group consisting ofpolybenzazole fiber, aromatic polyamide fiber, aromatic polyimide fiber,polyphenylene sulfide fiber, and wholly aromatic polyester fiber.Therefore, a highly thermally conductive carbon fiber can be produced inwhich graphite structure highly develops.

[0072] By pulverizing or cutting such carbon fiber, fine and uniformcarbon fiber powders can be produced that have two characters requiredfor thermally conductive filler: high thermal conductivity and highfilling capability.

[0073] The use of the graphitized carbon fiber powders that have aninterplanar spacing (d002) of graphite planes of less than 0.3370 nm byX-ray diffractometry makes it possible to produce highly thermallyconductive carbon fiber(s) or carbon fiber powders.

[0074] In the polymeric fiber having an aromatic ring on its main chain,linear polymers run principally in the direction of the fiber axis.Thus, the carbon fiber or carbon fiber powder can be produced in whichgraphite planes highly develop in the running direction of the linearpolymers or in the direction of the fiber axis. Consequently, the carbonfiber or carbon fiber powder has excellent thermal conductivity,particularly in the direction parallel to the graphite planes.

[0075] To produce the carbon fiber powder, the polymeric fiber having anaromatic ring on its main chain, preferably, at least one polymericfiber selected from the group consisting of polybenzazole fiber,aromatic polyamide fiber, aromatic polyimide fiber, polyphenylenesulfide fiber, and wholly aromatic polyester fiber, is heated at least2500 degree under vacuum or inert gas to graphitize. This produces ahighly thermally conductive carbon fiber in which the graphite structureis highly developed. By pulverizing or cutting this carbon fiber, fineand uniform carbon fiber powders that have good filling capability canbe produced simply. In addition, conventional infusibilization processis not necessary required. Moreover, by not pulverizing or cuttingplanar base material (graphite film) but by using a linear basematerial(carbon fiber), fine and uniform carbon fiber powders can beproduced easily. Thus, pulverization or cutting process is simplified,which results in improvement in productivity of the powders at amanufacturing step.

[0076] The carbon fiber powders are mixed in the matrix. Therefore, athermally conductive complex material can be obtained in which thecontent of the carbon fiber powders is larger and thermal conductivityis greater than conventional ones, or a thermally conductive complexmaterial can be obtained in which the content of the carbon fiberpowders is smaller but thermal conductivity is similar. When theinterplanar spacing (d002) of graphite planes by X-ray diffractometry isless than 0.3370 nm, the thermal conductivity of the thermallyconductive composition is greatly improved. Although the reason is stillunclear why the thermal conductivity is improved like this, it issupposed that it is ascribable to the very strong correlation betweenthe microstructure of the carbon fibers and thermal passages when thecarbon fibers are dispersed in the matrix.

[0077] As a matrix of the thermally conductive composition, at least onepolymeric material is used selected from the group consisting ofvulcanized rubber, thermoplastic elastomer, thermoplastic resin, andthermosetting resin that has molding capability. Thus, a thermallyconductive composition can be obtained that is applicable depending onvarious applications or required characteristics.

[0078] The content of the carbon fiber powders in the composition is 1to 800 parts by weight relative to 100 parts by weight matrix. Thisprevents an increase in the viscosity of the composition and preventsinclusion of bubbles. A thermally conductive composition is obtained inwhich the carbon fiber powders are uniformly dispersed in the matrix andwhich has improved thermal conductivity.

EXAMPLES

[0079] The above-mentioned embodiments are further described withreference to Samples, Examples and Comparative examples, which are notintended to limit the scope of the present invention in any way.

Sample 1, Carbon Fiber Powders

[0080] As a polymeric fiber having an aromatic ring on its main chain,polybenzazole fiber (Toyobo Co., Ltd., a tradename ZYLON™ HM:polybenzooxazole fiber) was used. After being bundled, the polymericfibers were placed in a heating reactor and heated at 3000 degree C. inan argon gas for two hours to be graphitized to produce carbon fibers.The resultant carbon fibers were pulverized with a high-speed rotationmill to form carbon fiber powders (Sample 1). The carbon fiber powdershad a fiber diameter of 9 μm, an average particle size of 50 μm, and aninterplanar spacing (d002) between the graphite planes of 0.3360 nm byX-ray diffractometry.

Sample 2, Carbon Fiber Powders

[0081] As a polymeric fiber having an aromatic ring on its main chain,polybenzazole fiber (Toyobo Co., Ltd., a tradename ZYLON™ HM:polybenzooxazole fiber) was used. After being bundled, the polymericfibers were cut to an average length of 5 mm, placed in a heatingreactor, and heated at 3200 degree C. in an argon gas for two hours tobe graphitized to produce carbon fibers. The resultant carbon fiberswere further pulverized with a high-speed rotation mill to form carbonfiber powders (Sample 2). The carbon fiber powders had a fiber diameterof 9 μm, an average particle size of 25 μm, and an interplanar spacing(d002) between the graphite planes of 0.3358 nm by X-ray diffractometry.

Sample 3, Carbon Fiber Powders

[0082] As a polymeric fiber having an aromatic ring on its main chain,aromatic polyimide fiber (Inspec Fibres, a tradename P84) was used.After being bundled, the polymeric fibers were placed in a heatingreactor and heated at 3000 degree C. in an argon gas for two hours to begraphitized to produce carbon fibers. The resultant carbon fibers werepulverized with a high-speed rotation mill to form carbon fiber powders(Sample 3). The carbon fiber powders had a fiber diameter of 9 μm, anaverage particle size of 50 82 m, and an interplanar spacing (d002)between the graphite planes of 0.3364 nm by X-ray diffractometry.

Sample 4, Carbon Powders

[0083] As a polymeric film, an aromatic polyimide film (Du Pont-TorayCo., Ltd., a tradename KAPTON™ a thickness of 25 μm) was used. Thepolymeric film was placed in a heating reactor and heated at 3000 degreeC. in an argon gas for 2 hours to be graphitized to produce a graphitefilm. The resultant graphite film was pulverized with a high-speedrotation mill to form carbon powders (Sample 4). Although the carbonpowders had an average particle size of 45 μm, the shape and size ofeach particle is unequal and fine powders of the size of less than 5 μmand coarse powders of the size of more than 500 μm were mixedconsiderably. The interplanar spacing (d002) between the graphite planesby X-ray diffractometry was 0.3368 nm.

[0084] The fiber diameter, the average particle size, and theinterplanar spacing (d002) between the graphite planes by X-raydiffractometry of the carbon fiber powders and the carbon powders ofSamples 1 to 4 are shown in Table 1. TABLE 1 Sample 1 Sample 2 Sample 3Sample 4 raw material polybenzazole polybenzazole aromatic aromaticfiber fiber polyimide fiber polyimide film fiber diameter  9 μm  9 μm  9μm  9 μm average particle 50 μm 25 μm 50 μm 45 μm interplanar spacing0.3360 nm 0.3358 nm 0.3364 nm 0.3368 nm (d002)

[0085] As described below, Examples 1 to 7 and Comparative examples 1and 2 are the examples of a soft thermally conductive sheet formed ofsilicone rubber. Examples 8 to 12 and Comparative example 3 are theexample of a thermally conductive sheet in which recyclablethermoplastic elastomer is used. Examples 13 to 17 and Comparativeexample 4 are example of a thermally conductive molded article that iscapable of injection molding. Examples 18 to 22 and Comparative example5 is a thermally conductive adhesive of epoxy resin.

Example 1

[0086] 90 parts by weight of the carbon fiber powders of Sample 1 withtheir surfaces treated with a silane coupling agent as a thermallyconductive filler, 220 parts by weight of aluminum oxide powder (SHOWADENKO K. K.), and 80 parts by weight of aluminum hydroxide powder (SHOWADENKO K. K.) were added to and dispersed in 100 parts by weight of anaddition-type liquid silicone rubber (Dow Corning Toray Silicone Co.,Ltd.) as a matrix to prepare a thermally conductive composition. Theresultant composition was hot-pressed and molded to form a thermallyconductive sheet of a thickness of 2 mm. The resultant sheet had anAsker C hardness of 17. The thermal conductivity in the thicknessdirection of the sheet was 2.9W/(m·K).

Example 2

[0087] A thermally conductive composition was prepared as in Example 1,except that the carbon fiber powders of Sample 2 were used as thermallyconductive filler. A thermally conductive sheet having a thickness of 2mm was formed. The resultant sheet had an Asker C hardness of 15. Thethermal conductivity in the thickness direction of the sheet was3.0W/(m·K).

Example 3

[0088] A thermally conductive composition was prepared as in Example 1,except that the carbon fiber powders of Sample 3 were used as thermallyconductive filler. A thermally conductive sheet having a thickness of 2mm was formed. The resultant sheet had an Asker C hardness of 15. Thethermal conductivity in the thickness direction of the sheet was2.7W/(m·K).

Comparative Example 1

[0089] A thermally conductive composition was prepared as in Example 1,except that the powders of Sample 4 were used as thermally conductivefiller. A thermally conductive sheet having a thickness of 2 mm wasformed. As described previously, the carbon powders of Sample 4 wereunequal so that the carbon powders are difficult to mix in the matrixand gas bubbles could not be removed. The resultant sheet had an Asker Chardness of 13. The thermal conductivity in the thickness direction ofthe sheet was 1.0W/(m·K).

Comparative Example 2

[0090] A thermally conductive composition was prepared as in Example 1,except that commercially available pitch-based carbon fiber powder(Petoca materials Ltd., milled Melblon) was used as thermally conductivefiller. A thermally conductive sheet having a thickness of 2 mm wasformed. The resultant sheet had an Asker C hardness of 15. The thermalconductivity in the thickness direction of the sheet was 2.3W/(m·K).

[0091] The contents of the thermally conductive composition of Examples1 to 3 and Comparative examples 1 and 2 and the thermal conductivity inthe thickness direction of their respective thermally conductive sheetsare shown in Table 2. TABLE 2 Ex. 1 Ex. 2 Ex. 3 Comp. 1 Comp. 2thermally conductive filler (type) Sample 1 Sample 2 Sample 3 Sample 4Pitch-based thermally conductive filler (pbw) 90 90 90 90 90 siliconerubber(pbw) 100 100 100 100 100 aluminum oxide (pbw) 220 220 220 220 220aluminum hydroxide (pbw) 80 80 80 80 80 thermal conductivity W/(m · K)2.9 3.0 2.7 1.0 2.3

[0092] A thermally conductive composition was prepared as in Example 1,except that the content of the carbon fiber powders was varied to 5, 20,300, and 500 parts by weight, respectively. A thermally conductive sheethaving a thickness of 2 mm was formed. The thermal conductivity in thethickness of the resultant sheets was 1.6W/(m·K), 199W/(m·K),3.6W/(m·K), and 4.2W/(m·K), respectively.

[0093] The contents of the thermally conductive composition of Example1, Examples 4 to 7, and Comparative example 1 and 2 and the thermalconductivity in the thickness direction of their respective thermallyconductive sheets are shown in Table 3. TABLE 3 Ex. 1 Ex. 4 Ex. 5 Ex. 6Ex. 7 Comp. 1 Comp. 2 thermally conductive filler (type) Sample 1 Sample1 Sample 1 Sample 1 Sample 1 Sample 4 Pitch-based thermally conductivefiller (pbw) 90 5 20 300 500 90 90 silicone rubber(pbw) 100 100 100 100100 100 100 aluminum oxide (pbw) 220 220 220 220 220 220 220 aluminumhydroxide (pbw) 80 80 80 80 80 80 80 thermal conductivity W/(m · K) 2.91.6 1.9 3.6 4.2 1.0 2.3

Example 8

[0094] 100 parts by weight of low-hardness styrenic thermoplasticelastomer (RIKEN VINYL INDUSTRY CO., LTD.) as a matrix, 100 parts byweight of the carbon fiber powders of Sample 1 as a thermally conductivefiller, 20 parts by weight of boron nitride powder (DENKI KAGAKU KOGYOKK.), and 20 parts by weight of aluminum hydroxide powder (SHOWA DENKOK.K.) were mixed with a two-axis extruder to prepare a pellet-likethermally conductive composition. The resultant composition wasextrusion-molded to form a thermally conductive sheet of a thickness of3 mm. The resultant sheet had a Shore A hardness of 73. The thermalconductivity in the thickness direction of the sheet was 2.1W/(m·K).

Examples 9 to 12

[0095] A pellet-like thermally conductive composition was prepared as inExample 8, except that the content of the carbon fiber powders wasvaried to 5, 20, 300, 500 parts by weight, respectively. A thermallyconductive sheet having a thickness of 3 mm was formed. The thermalconductivity in the thickness of the resultant sheets was 1.3W/(m·K),1.6W/(m·K), 2.9W/(m·K), and 3.6W/(m·K), respectively.

Comparative Example 3

[0096] A pellet-like thermally conductive composition was prepared as inExample 8, except that commercially available pitch-based carbon fiberpowder (Petoca materials Ltd., milled Melblon) was used as thermallyconductive filler. A thermally conductive sheet having a thickness of 3mm was formed. The resultant sheet had a Shore A hardness of 68. Thethermal conductivity in the thickness of the sheet was 1.4W/(m·K).

[0097] The contents of the thermally conductive composition of Examples8 to 12 and Comparative example 3 and the thermal conductivity in thethickness direction of their respective thermally conductive sheets areshown in Table 4. TABLE 4 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Comp. 3thermally conductive filler (type) Sample 1 Sample 1 Sample 1 Sample 1Sample 1 Pitch-based thermally conductive filler (pbw) 100 5 20 300 500100 thermoplastic elastomer 100 100 100 100 100 100 boron nitride(pbw)20 20 20 20 20 20 aluminum hydroxide (pbw) 20 20 20 20 20 20 thermalconductivity W/(m · K) 2.1 1.3 1.6 2.9 3.6 1.4

Example 13

[0098] 100 parts by weight of polyacetal resin (Asahi Kasei Corporation)as a matrix, 80 parts by weight of the carbon fiber powders of Sample 2with their surfaces treated with a silane coupling agent as a thermallyconductive filler, and 50 parts by weight of aluminum oxide powder(SHOWA DENKO K.K.) were mixed with a two-axis extruder to prepare apellet-like thermally conductive composition. The resultant compositionwas injection-molded to form a thermally conductive molded article of athickness of 3 mm. The thermal conductivity in the thickness directionof the resultant molded article was 1.7W/(m·K).

Examples 14 to 17

[0099] A pellet-like thermally conductive composition was prepared as inExample 13, except that the content of the carbon fiber powders wasvaried to 5, 20, 300, 500 parts by weight, respectively. A thermallyconductive molded article was formed having a thickness of 3 mm. Thethermal conductivity in the thickness of the resultant molded articleswas 1.1W/(m·K), 1.3W/(m·K), 2.8W/(m·K), and 3.5W/(m·K), respectively.

(Comparative example 4

[0100] A pellet-like thermally conductive composition was prepared as inExample 13, except that the carbon powders of Sample 4 were used asthermally conductive filler. A thermally conductive molded article wasformed having a thickness of 3 mm. As described previously, the carbonpowders of Sample 4 were unequal so that the carbon powders aredifficult to mix in the matrix and gas bubbles could not be removed. Thethermal conductivity in the thickness of the resultant molded articlewas 0.8W/(m·K).

[0101] The contents of the thermally conductive composition of Examples13 to 17 and Comparative example 4 and the thermal conductivity of thethermally conductive molded articles are shown in Table 5. TABLE 5 Ex.13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Comp. 4 thermally conductive filler(type) Sample 2 Sample 2 Sample 2 Sample 2 Sample 2 Sample 4 thermallyconductive filler (pbw) 80 5 20 300 500 80 polyacetal resin (pbw) 100100 100 100 100 100 aluminum oxide (pbw) 50 50 50 50 50 50 thermalconductivity W/(m · K) 1.7 1.1 1.3 2.8 0.8 0.8

Example 18

[0102] 150 parts by weight of the carbon fiber powders of Sample 1 withtheir surfaces treated with a silane coupling agent as a thermallyconductive filler and 30 parts by weight of aluminum oxide powder (SHOWADENKO K.K.) were added to and dispersed in 100 parts by weight ofbis-phenol F-type epoxy resin (YUKA SHELL EPOXY CO., LTD) containingamine-type hardener as an adhesion polymer matrix to prepare a thermallyconductive composition which is a thermally conductive adhesive. Theresultant composition was thermally hardened to form a plate specimen ofthickness of 1 mm. The thermal conductivity of the resultant platespecimen was 2.9W/(m·K).

Examples 19 to 22

[0103] A thermally conductive composition, which is a thermallyconductive adhesive, was prepared as in Example 18, except that thecontent of the carbon fiber powders was varied to 5, 20, 300, 500 partsby weight, respectively. A plate specimen of thickness of 1 mm wasformed. The thermal conductivity in the thickness of the resultant platespecimen 1.2W/(m·K), 1.8W/(m·K), 3.4W/(m·K), and 4.1W/(m·K),respectively.

Comparative example 5

[0104] A thermally conductive composition, which is a thermallyconductive adhesive, was prepared as in Example 18, except thatcommercially available pitch-based carbon fiber powder (Petoca materialsLtd., milled Melblon) was used as thermally conductive filler. A platespecimen of thickness of 1 mm was formed. The thermal conductivity ofthe resultant plate specimen was 2.3W/(m·K).

[0105] The contents of Examples 18 to 22 and Comparative example 5 andthe thermal conductivity of the thermally conductive adhesives are shownin Table 6. TABLE 6 Ex. 18 Ex. 19 Ex. 20 Ex. 21 Ex. 22 Comp. 5 thermallyconductive filler (type) Sample 1 Sample 1 Sample 1 Sample 1 Sample 1Pitch-based thermally conductive filler (pbw) 150 5 20 300 500 150 epoxyresin (pbw) 100 100 100 100 100 100 aluminum oxide (pbw) 30 30 30 30 3030 thermal conductivity W/(m · K) 2.9 1.2 1.8 3.4 4.1 2.3

Discussion

[0106] As described above, all the thermally conductive compositions ofExamples 1 to 22 include, in the matrix, the carbon fiber powders of thepresent invention (Sample 1 to 3) that are obtained by graphitizing thepolymeric fiber having an aromatic ring on its main chain by heating.

[0107] On the other hand, the thermally conductive compositions ofComparative example 1 and Comparative example 4 include, in the matrix,the conventional carbon powders (Sample 4) that are obtained bygraphitizing the aromatic polyimide film by heating and pulverizing it.

[0108] The thermally conductive compositions of Comparative example 2,Comparative example 3 and Comparative example 5 also include, in thematrix, the conventional pitch-based graphitized carbon fiber powdersthat are produced by treating raw pitch by several processes such asspinning, infusibilization, graphitization, and pulverization.

[0109] The compositions of Examples 1 to 3 had higher thermalconductivity compared with those of Comparative example 1 andComparative example 2. The compositions of Example 8 had a higherthermal conductivity than that of Comparative example 3. Thecompositions of Example 13 had a higher thermal conductivity than thatof Comparative example 4. The composition of Example 18 had a higherthermal conductivity than that of Comparative example 5. Accordingly, iswas confirmed that the carbon fiber powders of the present invention ofSamples 1 to 3 had a higher thermal conductivity than that of theconventional carbon powders of Sample 4 or the conventional pitch-basedcarbon fiber powders.

[0110] As described previously, the carbon powders of Sample 4 wereunequal so that the carbon powders are difficult to mix in the matrixand gas bubbles could not be removed (See Comparative example 1 andComparative example 4). As appreciated from this fact, it was confirmedthat the carbon powders of Sample 4 had poor filling capability. Inaddition, the resultant thermally conductive compositions had poorthermal conductivity (See Comparative example 1 and Comparativeexample). On the other hand, such tendency could not be observed whenthe content of the carbon fiber powders of Samples 1 to 3 was varied ina range from 5 to 500 parts by weight. Instead, it was confirmed thatthe resultant thermally conductive compositions had high thermalconductivity (See Examples 1 to 22). Thus, it was confirmed that thecarbon fiber powders of the present invention of Samples 1 to 3 werealso superior in filling capability to the conventional carbon powdersof Comparative example 4.

[0111] In the method of making the carbon fiber powders (Samples 1 to3), the fiber is heated, graphitized, and pulverized or cut. Thiseliminates the need of an infusibilization process. In addition, byusing carbon fibers rather than a film, the pulverization or cuttingprocess is simplified. Therefore, in this method, high-performancecarbon fiber powders that have two characters of high thermalconductivity and high filling capability can be manufactured easily.

[0112] It should be apparent to those skilled in the art that thepresent invention may be embodied in many other specific forms withoutdeparting from the spirit or scope of the invention.

[0113] Therefore, the present examples and embodiments are to beconsidered as illustrative and not restrictive and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalence of the appended claims.

1. A carbon fiber produced by graphitizing a polymeric fiber having anaromatic ring on its main chain by heating.
 2. The carbon fiberaccording to claim 1, wherein the polymeric fiber is selected from thegroup consisting of polybenzazole fiber, aromatic polyamide fiber,aromatic polyimide fiber, polyphenylene sulfide fiber, and whollyaromatic polyester fiber.
 3. The carbon fiber according to claim 1,wherein the carbon fiber has an interplanar spacing (d002) of graphiteplanes of less than 0.3370 nm by X-ray diffractometry.
 4. A method ofmaking a carbon fiber comprising: graphitizing a polymeric fiber havingan aromatic ring on its main chain by heating the polymeric fiber atleast 2500 degree C. under vacuum or in an inert gas.
 5. The methodaccording to claim 4, wherein the polymeric fiber is selected from thegroup consisting of polybenzazole fiber, aromatic polyamide fiber,aromatic polyimide fiber, polyphenylene sulfide fiber, and whollyaromatic polyester fiber.
 6. A carbon fiber powder produced bygraphitizing a polymeric fiber having an aromatic ring on its main chainby heating.
 7. The carbon fiber powder according to claim 6, wherein thepolymeric fiber is selected from the group consisting of polybenzazolefiber, aromatic polyamide fiber, aromatic polyimide fiber, polyphenylenesulfide fiber, and wholly aromatic polyester fiber.
 8. The carbon fiberpowder according to claim 6, wherein the carbon fiber powder has aninterplanar spacing (d002) of graphite planes of less than 0.3370 nm byX-ray diffractometry.
 9. The carbon fiber powder according to claim 6,wherein the fiber powder has a diameter of from 5 to 20 μm and anaverage particle size of from 5 to 500 μm.
 10. A method of making acarbon fiber powder comprising: graphitizing a polymeric fiber having anaromatic ring on its main chain by heating the polymeric fiber at least2500 degree C. under vacuum or in an inert gas.
 11. The method accordingto claim 10 further comprising: pulverizing or cutting the polymericfiber after the graphitizing step.
 12. The method according to claim 10,wherein the polymeric fiber is selected from the group consisting ofpolybenzazole fiber, aromatic polyamide fiber, aromatic polyimide fiber,polyphenylene sulfide fiber, and wholly aromatic polyester fiber.
 13. Athermally conductive composition comprising: a matrix; and carbon fiberpowders mixed in the matrix, wherein the carbon fiber powder is producedby graphitizing a polymeric fiber having an aromatic ring on its mainchain by heating.
 14. The composition according to claim 13, wherein thematrix has at least one polymeric material selected from the groupconsisting of vulcanized rubber, thermoplastic elastomer, thermoplasticresin, and thermosetting resin.
 15. The composition according to claim13, wherein the content of the carbon fiber powders is from 1 to 800parts by weight relative to 100 parts by weight of the matrix.
 16. Thecomposition according to claim 13, wherein the composition is athermally conductive molded article molded into a predetermined shape.17. The composition according to claim 13, wherein the composition is athermally conductive sheet.
 18. The composition according to claim 13,wherein the composition is a thermally conductive adhesive.
 19. Thecomposition according to claim 13, wherein the composition is athermally conductive grease.